1. Field of the Invention
The present invention relates to a process for the setting of a constant temperature for a fluid, and furthermore, relates to a thermostat for implementing the process. The constant temperature is set by the heating up of the fluid during its flow through a heat source.
An extremely exact temperature regulation is required for fluids with a low mass or, in effect, low flow velocity; for instance, for the determination of the output capacity of solar collectors. The fluids flow through the solar collectors and are, as a consequence, heated up. The solar collectors are heating units which belong to the low-temperature heating technology, wherein during the heating up in solar collectors, these provide for only relatively low temperature differences between the inlet and outlet temperature of the fluid. In order to determine the dissipation loss and the operating efficiency of solar collectors, it is necessary to provide for extremely exactly set inlet temperatures of the fluid which is to be heated up within the solar collector. Pursuant to German Industry Standard DIN 4757/4, the inlet temperature is permitted to maximally deviate by only about .+-.0.1 K from a pregiven rated temperature.
Consequently, the fluid is either withdrawn from a bath thermostat with a suitable fluid reservoir, or a heat exchanger is employed as thermostat, through which the required fluid quantity is set to the rated temperature by means of a control circuit.
2. Discussion of the Prior Art
In order to determine the operating efficiency of solar collectors, the fluid quantity which is heated up in the solar collector is conducted in a closed circuit on test stands. The solar collector is measured under differently intense and, upon occasion, fluctuating solar radiation; in essence, the fluid is heated up to a different extent within the solar collector. In such test stands, the thermostat, as a result, is not only adapted for the heating up of the fluid prior to its inlet into the solar collector, but also for effectuating a cooling to the pregiven rated temperature at the inlet of the solar collector. It is disadvantageous that the fluid, which is differently heated after passing through the test section, necessitates during its reconveyance in a closed circuit in thermostats, a differently intense heating up or cooling down in order to be heated or cooled to the rated temperature at the inlet of the solar collector. As a result, the thermostats possess an extremely complex setting arrangement in order to be able to maintain the prescribed maximum deviation of the inlet temperature of the fluid in the solar collector at about .+-.0.1 K.